It uses different tools depending on the type of partition, taken from their website:

Based on Partclone (default), Partimage (optional), ntfsclone (optional), or dd to image or clone a partition.

It will use them in that order typically to try and copy your partition. dd is a last resort because it is just sector by sector copying and does not have any optimization built in that would be based on the filesystem type of the partition. For example, cloning an ntfs partition would be a lot fast than cloning a hfs partition (at least with an older version of clonezilla, haven't used it in a while) because there was no built in tool for efficient hfs+ copying, and it used dd.